Hybrid vehicle transmissions known from practice have various design configurations depending on the vehicle weight, the vehicle load, trailer operation, the desired gradeability of a vehicle designed with a hybrid vehicle transmission, driving over a curb edge in pure electric mode, and the like. So, for example, in vehicles having a high vehicle weight, an electric machine of a hybrid vehicle transmission or the rotor thereof is connected to the transmission input shaft by means of a planetary transmission having a constant transmission ratio of approximately 1.7. For vehicles having a low vehicle weight, the electric machine or the rotor thereof is coupled directly to the transmission input shaft at a transmission ratio equal to 1.
Additional drive motors of hybrid drive trains, often designed as internal combustion engines, can be connected to the transmission input shaft by means of a shift element, wherein disadvantageously only the entire drive can be disconnected from the transmission input shaft for the hybrid vehicle transmissions described above, for which reason the vehicle cannot be driven by means of the internal combustion engine alone.
Furthermore, because of the hybrid vehicle transmission variant designed with the constant transmission ratio, a drive-off torque electrically generated in the region of the electric machine cannot be provided in the region of the transmission input shaft at the level of a drive-off torque that can be provided by means of a conventional transmission for a vehicle driven by an internal combustion engine.
Therefore, a so-called emergency drive-off of a vehicle with a connected internal combustion engine is required, depending on a loading state of the vehicle, a grade presently to be driven through, a curb edge, or the like, although an electric storage device associated with the electric machine has a sufficient state of charge. However, such emergency drive-offs are undesired, because they increase the fuel consumption of a hybrid vehicle to an undesired extent.
In addition, for the hybrid vehicle transmission variants described above, a rotor of an electric machine cannot be uncoupled from the transmission input shaft, so that during cross-country drives during which an electric storage medium associated with the electric machine is fully charged and during which a vehicle is driven substantially only by the internal combustion engine, the rotor mass of the electric machine must also be continuously moved or accelerated, whereby in turn the fuel consumption is likewise increased.
In addition, the maximum speed of a vehicle designed with a hybrid vehicle transmission described above is limited due to a permissible maximum rotational speed of the electric machine. This results from the fact that the permissible maximum rotational speed of a common electric machine is approximately 4,500 revolutions per minute. However, at an internal combustion engine rotational speed of 2,800 revolutions per minute, which substantially corresponds to the rotational speed of the transmission input shaft, and a constant transmission ratio of 1.7 between the transmission input shaft and the rotor, the rotor of the electric machine is driven at a rotational speed of 4,760 revolutions per minute, so that the permissible maximum rotational speed of a common electric machine is exceeded.